This invention relates generally to positioning equipment and particularly to a system for verifying the positioning of a movable table within a narrow distance range.
The picture tube for many color television receivers includes a phosphor screen which is composed of alternating stripes of phosphor, each of which emits a different color of light when impacted by electrons. The phosphor stripes are produced on a faceplate panel by coating the inside surface of the panel with a slurry containing a photosensitive material and one of the phosphors. An apertured shadow mask is inserted into the panel and the panel is placed upon a movable table of an exposure mechanism, commonly called a lighthouse. The lighthouse includes a bright light source which is used to expose the slurry to light. The shadow mask contains a large number of apertures and accordingly during the exposure the faceplate panel is moved so that the slurry portions shaded by the metal between the apertures are exposed and continuous lines are produced on the screen. After the exposure is completed the panel is removed from the movable lighthouse table. The shadow mask is removed and a slurry containing the photosensitive material and another phosphor is coated onto the panel. The shadow mask is reinserted and the panel is again placed onto the movable lighthouse table. The lighthouse table is returned to the original starting position prior to the panel being put back onto the lighthouse. U.S. Pat. No. 4,370,036 describes a system for controlling a lighthouse which operates in this manner.
The locations of the phosphor stripes on the screen relative to each other, and with respect to the shadow mask, are very important. Also, the lensing for the exposing light is different for each of the three color producing phosphor to simulate the paths of the electron beams toward the phosphor screen. For these reasons, it is important that the lighthouse table very accurately return to the original starting position prior to receiving a panel. When the movable lighthouse table is not accurately returned to the original starting position, within a narrow tolerance range, an unusable screen usually is produced. Accordingly, there is a need for a system for verifying the positioning of a movable table within a very precise distance range. The present invention fulfills this need.